All
of the known butterfly colonies in Mexico are measured once each
winter. Like a snapshot in time, these annual winter measurements
give scientists a chance to estimate the size of the entire overwintering
population.

The
graph above shows each winter's population estimate
since the winter of 1994/1995.
Since that time, when was the population the highest? The lowest?
How do this year's numbers compare?

What
is a monarch colony? Look at the photo to the right. The orange
you see in certain trees are monarch butterflies! This is a monarch
colony. Find out how scientists measure and estimate monarch numbers.